The invention concerns a downhole tool cooling device wherein a downhole tool is thermally coupled to a rechargeable cold source comprising a solid cold source body being contained in an insulated cooling medium vessel, and wherein the downhole tool is thermally coupled to the cold source by means of a cooling circuit comprising a first heat exchanger arranged at the downhole tool and in a fluid communicating manner being interconnected with a second heat exchanger arranged in the solid cold source body. Furthermore it concerns a method for cooling a downhole tool, and finally the invention concerns use of a solid cold source body contained in an insulated cooling medium vessel as a cold source for a cooling circuit being thermally coupled to a downhole tool being in the need of cooling during downhole operations.
Oil-well logging tools are by definition built to work in a hostile environment. This means that they need to operate at temperatures and pressures, which are significantly higher than those encountered in everyday usage of electronic equipment. Methods describing cooling of electronic components using Peltier elements have been disclosed in the past. Thermoelectric systems generally use Peltier elements, which are capable of moving thermal energy from one side of their envelope to the opposite side with application of an electrical voltage, creating quite high differences in temperature from one side to the other. Such systems are most commonly found in PCs, for example, to assist in the cooling of the central processing unit. The issue with Peltier elements is that their effective efficiency i.e. the amount of energy consumed compared to the amount of energy moved between the hot and cold surface can fall to very low values, such as less than 2% efficiency, when high differences in temperatures across the elements are required. In hot environments, such as exploration or production boreholes for oil and gas, the environmental temperatures can be in excess of 200° C. Electronics generally have a maximum operating temperature of 70-80° C. (for processors), and even automotive electronics can only function below 150° C. In such cases the required temperature difference which a system has to be capable of achieving to ensure that a device remains below 70° C. can be as high as 130° C. In this respect, at such high temperatures, if a Peltier element was employed to transport 10 watts of thermal energy away from a device by depositing said thermal energy into a hot environment of 175° C., for example, then at 2% efficiency, the Peltier element would consume 500 W of power in the process. In reality, such elements are usually rated for power consumption levels much lower than this, so the effective efficiency losses results in the inability of the system to maintain the cold-end cold.
In the example of borehole exploration drilling and oil and gas production systems, where devices such as instruments, mechanical or electronic items need to be maintained at a temperature much lower than that of the surrounding environment, such a power consumption would be impractical, as most power conveyance systems (such as wireline cables) can only carry a maximum of 1000 W, for which the majority of the power is dissipated in the primary systems, and not in supporting systems such as cooling.
The refrigeration method usually consists of a single or series of linked compression and evaporator cycles, as best described by a standard domestic refrigerator. Although, such systems do not function well when the hot-end radiator is already hot as such systems rely on convection to remove the excess heat from the radiating element. In addition, the temperature difference required for maintaining an operating temperature for electronics in a hot environment, as depicted above, requires multiple stages of refrigerators each with a different working fluid. In this respect, standard Freon-type systems do not boast the operating temperature required for such applications, an additional issue is that refrigeration systems require compressors and a multitude of moving parts, with the consequent reduction in reliability and robustness.
In recent years, attempts have been made to use free piston Stirling engines in hot environments, such as exploration and production wells, with limited success. The systems rely upon the active driving of the compression piston only. The displacement piston is connected only to a spring for displacement and resonance. Such systems need to be tuned so that the entire assembly reciprocates in resonance, whereby the displacement piston oscillates in harmonic motion out of phase with the harmonic motion of compression piston. The compression piston may be oscillated by use of a linear actuator or copper-coil and magnet combination, or by mechanical arm connection to a rotating disk, as illustrated in the original Stirling engine. In this respect, such beta-cycle free-piston Stirling engines can be highly efficient as only one piston is being driven, with an effective reduction in mechanical or electrical load as a result.
However, the phase relationship between the compression piston and the displacer piston is a function of the resonant frequency of the system which is a function of the masses of the pistons, the compression ratios, the pressure of the working fluid and the temperature of the working fluid. As the temperature of the working fluid increases as a result of a hot external environment, the pressure of the working fluid changes too, the result is a change in the resonant frequency of the system which alters the phase relationship between the pistons. In practice, the trapezium form of the Carnot cycle decreases and diminishes as the phase angle of the two pistons decreases from the typical 60 degrees down to 0 degrees. In this respect a free-piston Stirling engine becomes less and less efficient as the working fluid changes temperature and pressure, in addition the cycle collapses and the phase relationship descends to a phase angle of zero degrees meaning that there is no bias between the hot and cold sides of the system. The free-piston Stirling engine requires that the hot-side is actively cooled in some way.
In the case of an application of the Stirling cooler technology within a borehole for exploration or production, the environment can be very hot (up to 175° C.). Cooling has to be done via convection to the borehole liquid(s), preferable while the downhole tool is moving. The Stirling cooler has to be laid out to function in these hot ambient conditions. It will transfer thermal energy at an overall efficiency of about 25% and as such allow the cooling of a sold source, which in turn is inside a Dewar flask.
US 2006/0144619 A1 describes an apparatus for circulation of a coolant through a thermal conduit thermally coupled to a chassis heat exchange element including a plurality of receiving sections thermally coupled to a corresponding plurality of electronic devices. The temperature of one or more of the plurality of electronic devices may be sensed, and the flow rate of the coolant adjusted in accordance with the sensed temperature. The thermal conduit may be placed in fluid communication with a heat exchanger, perhaps immersed in a material, such as a phase-change material, including a eutectic phase-change material, a solid, a liquid, or a gas. A variety of mechanisms can be used to cool the apparatus when it is brought to the surface after operation in the borehole. In some cases, it is desirable to remove and replace the apparatus entirely. In others, a charging pump is used. The charging pump may be used to circulate the coolant in the conduit of the apparatus. For rapid turnaround, the coolant may be chilled while it is circulated. This can occur either by replacing the coolant with new coolant, or simply chilling the existing coolant and circulating it within the conduit until the temperature of the circulated coolant remains at a selected temperature.
US2004/00264543 A1 describes a temperature management system for managing the temperature of a discrete, thermal component. The temperature management system comprises a heat exchanger in thermal contact with the thermal component. The system also comprises a fluid transfer device that circulates a coolant fluid through a thermal conduit system. As the coolant flows through the heat exchanger, it absorbs heat from the component. Upon exiting the heat exchanger, the heated coolant flows to the heat sink where the heat sink absorbs heat from the coolant fluid, the heat sink comprising a phase change material. Phase change material is designed to take advantage of the heat absorbed during the phase change at certain temperature ranges. For example, the phase change material may be a eutectic material having a component composition designed to achieve a desired melting point for the material. The desired melting point takes advantage of latent heat of fusion to absorb energy. When the material changes its physical state, it absorbs energy without a change in the temperature of the material. Therefore, additional heat will only change the phase of the material, not its temperature. To take advantage of the latent heat of fusion, the eutectic material would have a melting point below the boiling point of water and below the desired maintenance temperature of the thermal component.